The present invention relates to slitting stacks of sheet material made of corrugated paperboard and, more particularly, to an apparatus and method for slitting knocked down boxes made of corrugated paperboard and formed in a stack in a stacking device as the boxes exit from a folding and gluing apparatus.
Corrugated paperboard box blanks are referred to as "knocked down boxes" in a flexo-folder-gluer apparatus. This apparatus includes a flexographic printer, a folding mechanism which folds opposite sides of the blank along pre-scored lines, and a gluing device which applies an adhesive along the overlapping edges of the laterally folded sides. The flattened container or knocked down box is thus completely formed and, after the glue dries, the boxes can be stacked and banded for shipment and subsequent assembly. It is known in the art to stack the knocked down boxes exiting the flexo-folder-gluer (hereinafter sometimes referred to as a "flexo") to utilize the stack weight to hold the glued edges together until the glue sets. It is also known in the art to form a shingle of knocked down boxes as they exit from the flexo, also utilizing the weight of the overlapping boxes in the shingle to hold the box position until the adhesive dries.
The knocked down boxes typically assembled in a flexo are of a conventional construction, including four sides, the overlapping edges of two sides of which are glued together on a glue tab, and four slotted end flaps extending integrally from opposite ends of the sides to eventually form the top and bottom closure flaps when the box is subsequently assembled. As indicated, these knocked down boxes are ordinarily finished containers and require no further processing, apart from stacking and banding for shipment. However, it is also known in the art to assemble certain special constructions of knocked down boxes in a flexo, which boxes are subsequently slit into two or more parts to form smaller containers of either a conventional or modified type. For example, it is known to assemble a large regular slotted container (RSC) and subsequently slit the same along a median line to form two half slotted containers, each of which comprises a knocked down container with side walls and bottom flaps or top flaps, but not both. Similarly, a large special regular slotted container can be formed in a flexo in the form of two integrally attached half size regular slotted containers by forming the blank with special double length center slots which, when bisected as the large special RSC is subsequently slit in half perpendicular to the center slots, form the two half-size RSCs.
Although the formation of the foregoing types of large knocked down boxes, which must be subsequently slit for end use, is well known, production of such boxes on a large scale has never been achieved, primarily because of difficulties in slitting them. Corrugated paperboard sheet stock is conventionally slit longitudinally by the use of a pair of upper and lower cooperating slitting blades which operate as a shear-type cutter. It has been found, however, that such dual knife shear cutters do not provide clean cuts with heavy and/or multi-wall corrugated board. Shear-type slitting inherently causes a vertical displacement of the adjacent slit edges of the board and, as the board thickness increases or as multiple layers are slit, the relative vertical displacement becomes larger and a ragged cut edge typically results. The multiple board layers presented by a knocked down box result in the same characteristic ragged cuts when shear-type slitters are used.
In addition, slitting large special containers exiting a flexo-folder-gluer has typically been done as an off-line process. In other words, the large knocked down boxes are taken off the flexo, moved to another location, and slit individually to form two half-size knocked down boxes. Even with this technique, the longitudinal slits are typically less than satisfactory because of the use of shear-type slitting devices. In addition, registration of the boxes, meaning lateral alignment so that the slit is directly on the centerline of the large regular or special slotted container, is difficult to attain with conventional off-line methods in which one box at a time is slit.
Nevertheless, real advantages in production volume and box quality could be attained with an apparatus and method which would slit large regular or special slotted containers to form two half-size containers in an on-line basis. Furthermore, small containers are typically not run on a flexo because small container blanks are extremely difficult to handle, not only in the flexo, but in upstream material handling devices as well. Thus, there is a real need in the industry for a system which can provide for the manufacture of high quality small size knocked down boxes, but will also utilize a flexo-folder-gluer in its most effective and efficient manner.
In one known prior art method, the on-line slitting of knocked down boxes is accomplished by forming a shingle of the boxes as they exit the flexo, unshingling the boxes downstream and feeding them one at a time through a conventional shear-type slitter, and then separately reshingling or stacking each of the series of half-size boxes. However, this process is slow, causes loss of box registration, and still results in ragged slit edges on the boxes.
It is also known to form knocked down boxes from a flexo-folder-gluer into a shingle and to slit the shingle on-line using a single thin high speed rotary slitting blade. Various techniques for slitting corrugated boxes in this manner are shown in U.S. Pat. Nos. 5,158,522 and 5,165,314. The apparatus for slitting such boxes is more broadly described in U.S. Pat. No. 5,090,281. Although high speed slitting with a single rotary slitting blade has improved substantially the quality of cuts, as well as processing speeds, excessive box handling equipment and steps are still required.
In accordance with the invention described in copending and commonly assigned application Ser. No. 065,935 identified above, knocked down boxes from a flexo-folder-gluer are stacked in a conventional counter ejector and the entire stack is transferred into a linear blade slitter which cuts through the entire stack, leaving two stacks of smaller regular slotted containers of either special or conventional construction. It has been found that a thin cutting blade, properly supported and driven at a small enough angle through the stack, can readily cut through a stack of folded knocked down boxes if properly oriented to slit essentially one box at a time and to allow the cut halves to part sequentially as the blade passes through the stack. The downward force of the blade on the layers of corrugated paperboard comprising the boxes will crush the corrugated medium unless the blade is moved through the stack at a small acute angle with respect to the plane of the end face of the stack (the plane of the means used to support the stack). Various types of linear slitting blades may be used, including a thin flexible blade clamped in a rigid blade support to expose only a small edge portion which defines the required slitting depth. The blades may be moved through the stack for cutting on a linear track means mounted for reciprocal movement through the cutting and return strokes.
In another embodiment, the cutting blade may comprise a continuous flexible band which operates linearly in one direction through a rigid blade guide with the guide and moving blade operated to pass directly through the stack of boxes. In this embodiment, linear blade movement is preferably at a speed about ten times as great as the speed of movement of the blade through the stack in a direction normal to the linear blade movement. Band blade slitting apparatus including rigid adjustable blade guide mechanisms have been developed for use in other slitting applications as shown, for example, in U.S. Pat. No. 3,393,538.